The dismal state of American roadways is well documented as is the deficiency in the funding for taking corrective measures. One basic problem with roadways is that the materials used are to a large extent incompatible with and dissociate from one another in response to traffic, a process accelerated in the presence of water. This is particularly true for asphalt concrete roadways. But as will become clear hereinafter, the remedy will be applicable to Portland Cement Concrete (PCC) roadways as well.
Asphalt concrete is a mixture of graded rocks and sand with asphalt oils. The composition requires no joints because it is flexible and can easily move to accommodate thermal expansion and contraction. Asphalt concrete is easy to place and creates a smooth pavement that supports heavy loading.
Asphalt concrete is applied hot and is ready for traffic when compacted and cooled. Because of the low cost, the simplicity and speed of application, and short time for return to traffic, asphalt concrete is the dominant pavement for building roads, driveways, and parking lots.
Unfortunately, rock and sand combined with asphalt oils is an inherently unstable mix with a short service life compared to Portland cement concrete pavements. Asphalt oils are nonpolar and do not form a strong attraction to polar materials like rocks and sand. Water has a strong attraction to the polar aggregates and readily displaces the asphalt oils. It is for this reason that water flowing over asphalt concrete pavements rather quickly removes the asphalt oils to expose and ravel the rocks and sand.
But a more serious limitation is caused by the flow of asphalt oils because of the continuous movement which occurs under traffic loads and thermal expansion and contraction. Rock and sand specify gravity is 2.6. Asphalt oils specific gravity is 1.1. This large difference in specific gravity causes rocks and sand to sink and asphalt oils to rise.
The difference in expansion and contraction, heat capacity, and thermal conductivity between asphalt oils and rocks and sand creates internal forces to force asphalt oil flow. As the asphalt concrete pavement heats, oils with their low heat capacity warm quickly, liquefy and expand at a rate much greater than the expansion of rocks and sand. The path of least resistance for expanding flowable oils is up to the surface.
Oil on the surface is exposed to traffic, heat and cold, oxygen, wind, dirt and sand, ultraviolet light and water to volatilize, oxidize, wear away and emulsify the exposed oils into the environment.
Thermal expansion is followed by thermal contraction as the pavement cools at night. The spacing between the rocks and sand and the asphalt oils is reduced from oil loss to shrink and stiffen the pavement.
It is no surprise then that as asphalt concrete ages and the asphalt oil disperses, the pavement becomes hard and brittle. Cracks form and the pavement shrinks. After the first cracks appear, breakdown accelerates rapidly. As water flows through the cracks to the base, the support for the pavement washes away. In short order more cracks appear followed by more serious pavement distress including potholes.
Although each of the components of asphalt concrete pavement is inherently stable and not readily prone to breakdown, the combination of rocks and oil is unstable. The situation is similar to the case with liquids. Liquids, such as oil and water are not mutually compatible, and are only made compatible by emulsification. The emulsion appears stable and uniform but the components are doomed to early separation in the absence of a suitable emulsifying agent or surfactant.
It is clear that asphalt concrete similarly is a mix of incompatible components with widely different polarity destined to a short life together unless means of stabilizing the mixture is used.